docs: add MkDocs documentation with Read the Docs theme

[Copilot]

Adds a complete documentation site for Squid Storage using MkDocs with the built-in readthedocs theme.

Structure

• mkdocs.yml — site config, nav tree, theme
• docs/index.md — project overview, feature summary, quick links
• docs/getting-started.md — prerequisites, build, running all three components, local cluster example
• docs/architecture.md — component diagram, data-flow for file updates, consistency/replication model
• docs/components/{server,datanode,client}.md — internals per component (state maps, lifecycle, dual-socket design, etc.)
• docs/protocol.md — Squid Protocol wire format, full message reference, file transfer framing
• docs/configuration.md — all CLI parameters with defaults across Server, DataNode, Client
• docs/contributing.md — commit conventions, branching, versioning, changelog
• docs/requirements.txt — mkdocs>=1.6.0

CI

.github/workflows/docs.yml — deploys to GitHub Pages on push to main when docs/ or mkdocs.yml changes.

Preview locally

pip install mkdocs
mkdocs serve
# → http://127.0.0.1:8000

site/ added to .gitignore.

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